1. Field of the Invention
The present invention generally relates to an organic electroluminescence (OEL) display device. More particularly, the present invention relates to organic electroluminescence display device having selective light valve.
2. Description of Related Art
In recent years, communication appliance has become more and more important. Especially, portable communication appliance is the major stream of the communication appliance. In the development of the portable communication appliance, the flat panel display (FPD) used for the communication interface between the user and the appliance is important. The conventional flat panel display (FPD) may be generally classified into plasma display panel (PDP), liquid crystal display (LCD), inorganic electro-luminescent display, light emitting diode (LED), vacuum fluorescence display and field emission display (FED). However, in comparison with other flat plane display device, the organic electroluminescence device has the advantages of self-luminescence, non-viewing angle dependence, power saving, easy process, low-cost, low temperature range of operation, high response speed and full-color. Therefore, the organic electroluminescence device has the potential to be the next generation flat panel display (FPD).
The organic electroluminescence component is used as a displaying device by using the characteristic of the self-luminescence of the organic functional material. The organic electroluminescence component is classified into small molecular organic light emitting component (SM-OLED) and polymer organic light emitting component (PLED) according to the molecular weight of the organic functional material. The structure for light emitting of the organic electroluminescence component is mainly constructed by a pair of electrodes and an organic functional material layer. When a current is applied between the pair of electrodes, exciton will be generated by the combination of electron and hole in the organic functional material layer. Thus, different color light is emitted by the exciton, and the color of the light emitted is dependent on the characteristic of the material of the organic functional material layer.
For any display device, the brightness discrimination is dependent on the ratio of the brightness between fully turned on and fully turned off, wherein the brightness ratio is generally called a contrast ratio (CR). The brightness discrimination is proportional to the contrast ratio. The conventional contrast ratio is generally defined as below:
                    CR        =                                            L                              sub                ,                on                                      +                          R              amb                                                          L                              sub                ,                off                                      +                          R              amb                                                          (        1        )            
In equation (1), Lsub,on is defined as the brightness of the pixel being fully turned on, Lsub,off is the brightness of the pixel being fully turned off, and Ramb is the reflected brightness of the external light being incident into the display device. When the Lsub,on and the Lsub,off of the pixel are known (for example, Lsub,on is about 100 nits and Lsub,off is about 1 nit), the relationship between the contrast ratio and the reflected brightness of the external light Ramb can be calculated according to equation (1). It is noted that, the larger the reflected brightness of the external light Ramb, the smaller the contrast ratio, i.e., the worse the brightness discrimination. However, the only way is to enhance the Lsub,on of the pixel to increase the contrast ration and obtain an applicable contrast ratio of the display device. Alternatively, the smaller the reflected brightness of the external light Ramb, the larger the contrast ratio, thus the Lsub,on of the pixel may be decreased to obtain an applicable contrast ratio. In general, the lower the Lsub,on of the pixel, the lower the power consumption and the lower harsh to the eyes.
FIG. 1 is a drawing schematically illustrating a conventional organic electroluminescence display device. Referring to FIG. 1, a conventional organic electroluminescence display device includes a substrate 100, a transparent electrode layer 102, an organic functional layer 104 and a metal electrode layer 106. The substrate 100 generally includes glass substrate. The transparent electrode layer 102 is generally composed of transparent conductive material such as indium tin oxide (ITO). The organic functional layer 104 is generally an organic multilayer comprising a hole injection layer, a hole transport layer, an organic electroluminescence layer, an electron transport layer and an electron injection layer. The metal electrode layer 106 is generally composed of aluminum, calcium or alloy of magnesium and silver. It is noted that, when current is applied between the transparent electrode layer 102 and the metal electrode layer 106, excitons will be generated by the recombination of electrons and holes in the organic functional layer 104 and light will be emitted. The mechanism of luminescence of different color of light is dependent on the characteristic of the material of the organic functional layer 104. In summary, the principle of displaying of the organic electroluminescence display device is to convert electric energy to photon energy by the driving of a current.
Referring to FIG. 1, the refractive index n1 of the organic functional layer 104 is very close to the refractive index n2 of the transparent anode layer 102. The refractive index n1 of the organic functional layer 104 is, for example, larger than the refractive index n3 of the transparent substrate 100. In general, n1 is about 1.7, n2 is between 1.8 and 2.0, and n3 is about 1.5 and larger than the refractive index of air (≈1).
In summary, the light of the organic electroluminescence display device is emitted by the organic functional layer 104, and the light has arbitrary propagation direction. However, the metal electrode layer 106 may be provided as a reflection layer so reflect the light, therefore, the light may only propagate in the direction from the metal electrode layer 106 to the substrate 100. In general, the emitted light described above may be influenced by the external light, and thus the brightness discrimination is reduced. For example, referring to FIG. 1, when the external light is incident to the organic electroluminescence display device, the external light will be reflected by the interface between the air and the transparent substrate 100, the interface between the transparent substrate 100 and the transparent electrode layer 102, and the interface between the organic functional layer 104 and the metal electrode layer 106. All the reflected light will propagate in the direction from metal electrode layer 106 to the substrate 100.
For example, referring to FIG. 1, the reflected light W1 from the interface between the air and the transparent substrate 100 is about 4% of the incident external light W. The reflected light W2 from the transparent substrate 100 and the transparent electrode layer 102 is about 0.8% of the incident external light W. The reflected light W3 from the organic functional layer 104 and the metal electrode layer 106 is over 90%. Therefore, almost all the reflected light is reflected by the metal electrode layer 106. In other words, almost all the reflected light is generated by the interface between the organic functional layer 104 and the metal electrode layer 106. Thus, if the organic electroluminescence display device is used outdoors with strong external light, the contrast ratio is reduced obviously due to the reflected external light. Therefore, how to reduce the incidence of the strong external light to the interface between the organic functional layer 104 and the metal electrode layer 106 is the most important issue to be solved to enhance the contrast ratio of the organic electroluminescence display device under strong external light.
One of the conventional technologies provides a polarizer attached to the surface of the display device, or provides a photo sensor and an adjusting device of contrast ratio to solve the problem described above. However, these two methods have the disadvantages below. First, the reflected external light is reduced by the polarizer, however, the light emitted by the device is also reduced by the polarizer, therefore the contrast of the display device is not enhanced and is not applicable for the display device to be used outdoors. Next, the photo sensor and the adjusting device of the contrast ratio are dependent on the intensity of the external light, thus the cost is high.